Vampires, Demons, Slaying, Oh my! Whedon-esque. Etc.

At the end of Season 5, Buffy jumps into the portal. But, as there's already a portal from Pylea opening up at the same time in order to bring back the AI team home, the frequency starts to get out of sync. Instead of the portal killing Buffy and leaving her body in Sunnydale, she jumps right through and into Pylea. And then straight through that into the Hyperion. The portal in Sunnydale was closed (as Buffy jumped through it and therefore closed it as the key was supposed to) however, she was never supposed to make it all the way through to LA. A dimensional rift is now rippling through the world and it's center point is LA. Other portals are slowly opening up and sending hell beasts and horrid things into the city. Other portals scoop people up and place them in alternate dimensions/realities. The Scoobie Gang come to help stop the catastrophe before it's too late and the world is consumed. Cue scenes.